The present invention relates generally to fluorescent polymeric articles that are light stable and that use a polymer matrix containing glycol-modified polyethylene terephthalate. The invention also relates to a method of making such light stable fluorescent articles.
Retroreflective sheeting is widely used for traffic and roadway safety signs. Such sheeting is typically provided as a polymeric monolayer or multilayer sheeting material having thousands of retroreflective elements, such as microprismatic corner cubes or glass microspheres that reflect incident light. It is well known to incorporate one or more fluorescent dyes into a retroreflective sheeting in order to enhance the visibility of articles such as road signs manufactured from such sheeting material. Fluorescent colors enhance visual contrast, which makes fluorescent colored materials more conspicuous than nonfluorescent materials. Unfortunately, most fluorescent colorants have poor ultraviolet light stability. In some cases, fading of fluorescent sheeting due to ultraviolet light exposure can occur within six months. The loss of fluorescence caused by ultraviolet light exposure dramatically shortens the useful life of fluorescent traffic and roadway signs. Accordingly, there is need in the art to stabilize fluorescent colorants in plastics and to find a means to reduce the fading of fluorescent dyes in order to provide articles such as retroreflective road signs that can remain in service for significantly longer periods.
To enhance the outdoor durability of fluorescent retroreflective sheeting, an ultraviolet light screening layer is often used to protect the fluorescent polymeric matrix layer from the effects of ultraviolet radiation. Traditionally, the U.V. light screening layer is made by incorporating U.V. light absorbing compounds into a transparent polymer matrix. Japan Kokai No. 2-16042, Application No. 63-165914 (Koshiji et al.) and U.S. Pat. No. 5,387,458 (Pavelka et al.) each disclose fluorescent articles consisting of an ultraviolet screen layer disposed in front of a fluorescent layer. According to these references, the screening layer contains substantial amounts of ultraviolet light absorbing compounds, which absorb a defined range of U.V. light (wavelength from about 290 to about 400 nm).
Such prior art multilayer structures in which a screening layer containing U.V. light absorbent additives is disposed in front of a layer containing a fluorescent dye can give rise to several difficulties. One problem is that the U.V. light absorbent additives incorporated into the U.V. light screening layer may leach out with time, because most U.V. light absorbing compounds are relatively small molecules and the U.V. light screening layer is typically quite thin. As a result of this phenomenon, the screening layer may lose its protective function, and the fluorescent colorants in the fluorescent layer will quickly fade and lose their fluorescence when exposed to ultraviolet light. A further problem with the U.V. light absorbent additive-treated screening layers is that U.V. light absorbing compounds present therein can diffuse or migrate into the fluorescent layer. If the U.V. light absorbing compound is not carefully selected, this diffusion can actually accelerate the fading of the fluorescent colorant even though the diffused compound is one that absorbs U.V. light. The problem of additive migration requires that a U.V. light absorbing additive incorporated into a screening layer be carefully matched to the fluorescent colorant so as to minimize any tendency of the migrating U.V. light absorber to affect the color and fluorescence of the colorant. The implication that one may randomly select any U.V. absorber capable of blocking most of U.V. light below about 400 nm wavelength (see, e.g., Japan Kokai No. 2-16042, Application No. 63-165914 (Koshiji et al.) and U.S. Pat. No. 5,387,458 (Pavelka et al)) fails to take into account the potential interaction between the U.V. absorber in the screening layer, and the fluorescent dye(s) present in the colored layer.
The use of a multi-layer polymeric structure also presents difficulties in manufacturing. Multiple films must be extruded or castand the individual films laminated together, resulting in a more expensive and more time-consuming process. Moreover, technical problems can arise. The different resins in a multi-layer structure must be compatible with one another, and must be processable within the same temperature range. Where the multi-layer article is a retroreflective sheeting structure and the refractive indices of contacting layers are different, the interface between the layers must be delicately controlled to optimize the optical characteristics of the resultant articles.
Other references disclose fluorescent sheeting articles which do not necessarily incorporate a screening layer and which have particular combinations of polymers and fluorescent dyes. Such references include U.S. Pat. No. 3,830,682 (Rowland), U.S. Pat. No. 5,605,761 (Burns et al.), U.S. Pat. No. 5,674,622 (Burns et al.), U.S. Pat. No. 5,672,643 (Burns et al.), U.S. Pat. No. 5,754,337 (Burns et al.), U.S. Pat. No. 5,920,429 (Burns et al.), and U.S. Pat. No. 6,110,566 (White et al.). Of these, only U.S. Pat. Nos. 5,605,761 and 6,110,566 present any data relating to fluorescent durability. This data indicates that the fluorescent durability of the structures disclosed is less than optimum. In particular, U.S. Pat. No. 5,605,761 discloses fluorescent articles comprising polycarbonate (PC), fluorescent dye, and a hindered amine light stabilizer. According to the reference, the combination of polycarbonate, fluorescent dye and hindered amine light stabilizer containing a 2,2,6,6-tetramethyl piperidine compound could extend the fluorescent lifetime of the resultant articles. U.S. Pat. No. 6,110,566 teaches that the combination of a fluorescent thioxanthene dye and a hindered amine light stabilizer in a solventless polyvinyl chloride (PVC) polymeric matrix will substantially enhance the light stability of the fluorescent colors in the PVC system. Neither of these patents, however, discloses structures which extend the life of the fluorescent color sufficiently for long-term use. With either system, within approximately 400 hours of accelerated artificial weathering, the data indicates that the fluorescent color has substantially shifted and the material begins to take on a faded appearance. This shift is indicated by the large loss, i.e. less than or equal to 50% dye retention, of the fluorescent dye from the either the PVC system or the PC system.
Based on the problems described above, there is a strong need in the art for fluorescent thermoplastic articles that exhibit improved color and/or fluorescent stabilization against ultraviolet radiation without requiring the placement of a separate ultraviolet light screening and/or absorbent layer over the article. In view of the foregoing, one object of the present invention is to provide a thermoplastic article in which a fluorescent dye is stabilized against ultraviolet light degradation in the absence of a separate ultraviolet light protectant layer.
Another object of the invention is to provide stabilized fluorescent retroreflective sheeting suitable for fabrication into outdoor weatherable products such as road signs, which are less susceptible to ultraviolet light weathering and the rapid loss of fluorescence resulting therefrom.
The present invention provides articles that exhibit unexpectedly durable fluorescence even after extended ultraviolet light exposure. In accordance with the invention, such articles comprise a polymeric matrix comprising poly(1,4-cyclohexanedimethanol-co-ethylene terephthalate), commonly referred to as glycol-modified polyethylene terephthalate, or PETG, and a fluorescent dye selected from the perylene imide and perylene ester dyes, thioxanthene dyes, benzoxanthene dyes, and benzothiazine dyes. The fluorescent dye must be thoroughly incorporated into the PETG resin system, preferably under conditions which create high shear but at temperatures that are not undesirably high. Ultraviolet light stabilization additives such as ultraviolet light absorbers (UVAs) or hindered amine light stabilizers (HALS) can also enhance the ultraviolet light stability of the resulting article. We have found that by careful selection of the fluorescent dyes and ultraviolet light stabilization additives, and by careful control of the process parameters used to disperse the fluorescent dye within the polymeric matrix, the fluorescent durability of the resultant article can be significantly enhanced beyond that which would have been expected in view of the prior art.
The invention is further directed to a retroreflective sheeting material comprising the polymeric article described above and having a plurality of retroreflective elements. Such retroreflective sheeting materials find particular utility when used to fabricate retroreflective road signs.
The fluorescent PETG articles of the instant invention exhibit enhanced fluorescent and color durability, yet without requiring the use of known UV light screening layers such as were used in the prior art. Because the UV light screening layer can be eliminated, there are no difficulties with regard to leaching out of UV light absorbers or migration of UV light absorbers from the screening layer which can actually cause accelerated fading of fluorescent colorants in a polymeric matrix. To those skilled in the art, it is evident that a PETG resin system itself is not extremely durable. If an extremely durable fluorescent PETG article is desired, an U.V. screening layer can be used to protect the PETG resin.
In accordance with the invention a fluorescent article comprises a polymeric matrix comprising poly(1,4-cyclohexanedimethanol-co-ethylene terephthalate) commonly referred to glycol-modified polyethylene terephthalate (or PETG), in which one or more fluorescent colorants and optional ultraviolet light stabilizing additives have been thoroughly dispersed. One class of suitable PETG resins is that which includes from about 80-95% by weight of a generally rigid polyethylene cyclohexanedimethylene terephthalate in which preferably from 2 to 20 molar parts of ethylene units are present per 1 molar part of cyclohexanedimethylene units. Such materials are generally described in U.S. Pat. No. 4,225,688 (Dennehey et al.) incorporated herein by reference in its entirety. PETG resins particularly suitable for use in the instant invention are Eastar GN-071 PETG, Eastar 6763 PETG, and Eastar UVSG PETG, all available from Eastman Chemical Company, Kingsport, Tenn. Other glycol-modified polyethylene terephthalates that might be suitable for use in the fluorescent articles of the instant invention include those disclosed in U.S. Pat. No. 5,955,565 (Morris et al.) also incorporated herein by reference in its entirety. Alloys of PETG and other resins also may be suitable.
We have discovered that four families of fluorescent dyes are particularly suitable for use in a PETG polymer resin system. These dye families include the perylene imide and perylene ester dyes such as Lumogen F Yellow 083, available from BASF Corporation (Rensselaer, N.Y.); thioxanthene dyes such as Solvent Yellow 98, available as Hostasol 3G from Clariant Corporation (Coventry, R.I.); benzoxanthene dyes such as Lumofast Yellow 3G, available from Day-Glo Color Corp (Cleveland, Ohio), and benzothiazine dyes such as Yellow 979, also known as Huron Yellow D-417, available from Day-Glo.
We have discovered that the conditions under which the fluorescent dye is incorporated into the PETG polymeric resin matrix significantly affect the fluorescence and color durability of the resultant PETG fluorescent articles. The fluorescent dye should be dispersed as thoroughly and evenly as possible within the PETG polymeric resin matrix. Furthermore, the temperature during the dispersion process must not be too high, yet significant shear must be imparted to the dye/PETG resin mixture in order to achieve adequate mixing of the dye into the resin matrix in accordance with the invention. On a laboratory scale, we have found that a Brabender prep-type mixer is suitable for preparing samples of fluorescent resin film in accordance with the instant invention. By comparison, a laboratory scale single-screw extruder run at traditional settings for zone and die temperatures, screw speed and take-up speed cannot provide the appropriate dwell time and shear required for processing fluorescent PETG articles of the instant invention. Though the fluorescent dye visually appears fully developed in film samples prepared using either method, the durability of films prepared using the prep-type mixer exceeds the durability of films prepared on a single-screw extruder. Not wishing to be bound to any particular theory, it is believed that the shear force, dwell time and processing temperature are important factors in producing extremely durable fluorescent PETG articles. It is believed that other laboratory scale mixing systems which provide high shear force and enough dwell time but which do not result in unduly high temperatures for prolonged periods of time also may be suitable for manufacturing films of the instant invention. Such mixing devices may include two-roll mills and twin-screw extruders. It is believed that a single-screw extruder run at nontraditional conditions might also produce fluorescent PETG materials with extreme durability if the temperature and screw speed are dropped in order to increase the shear force and dwell time. For production scale manufacturing, the shear force, dwell time, and processing temperature must be controlled in order to fully develop the fluorescent color in the PETG polymeric resin matrix in such a way as to produce excellent durability.